This invention relates generally to measuring electrical current, and in particular to measuring surge current.
A special form of digital multimeter known as a clampmeter includes a current sensing jaw that may be opened and placed around a wire to measure current. Conventional current clampmeters typically have a range switch for selecting a desired current range, and internal processing circuitry including a dual-slope analog-to-digital converter (ADC) for accurately measuring the value of current being measured. The final current reading is displayed on a liquid-crystal display (LCD) screen.
Accurate measurement of inrush surge current of an AC electric motor at startup would be useful because it reflects on the health of the motor, since a weak motor may draw excessive current. Conventional current clampmeters do not offer this feature.
Short duration surge currents have heretofore been extremely difficult to measure with any degree of accuracy for a number of reasons. While dual-slope ADCs provide extremely accurate measurements, particularly for DC measurements, the ADC operation is asynchronous to the surge current. Consequently, it is very difficult to capture the surge current as it occurs. Also, the measurement time of the dual-slope ADC is typically longer than the time duration of the surge current, or has dead time during which nothing is measured, resulting in faulty measurements.
For root-mean-square (RMS)xe2x80x94or average-responding instruments using analog detection methods, the time for the converter to respond makes such methods impractical for measuring motor inrush current. A very recently-produced current clampmeter instrument using an RMS-converter takes several seconds at lower current levels to reach final value.
It would be desirable to provide a clampmeter with an inrush current feature to measure current surges without resorting to expensive oscilloscope triggering and processing circuitry.
In accordance with the present invention, a novel method of and apparatus for accurately measuring surge currents such as motor-starting inrush currents is provided for a current-measuring instrument such as a clampmeter.
An input signal from a current sensor is monitored, and so long as the signal level is below a predetermined threshold, the display indicates that the current-measuring instrument is armed and ready to measure a surge current. When the input signal changes and exceeds the predetermined threshold, a surge current is detected, triggering the instrument, and the measurement process begins.
The input signal is acquired over a predetermined time period after the trigger point by a fast sampling ADC, which converts the input signal into a series of digitized samples representative of instantaneous current values. The absolute values of the digitized samples are accumulated (added together), and the total divided by the number of samples acquired to produce the average. The average is then formatted for display in terms of average current or RMS current, and finally displayed.
In a proposed commercial embodiment, a microcomputer with a fast ADC is utilized to carry out the measurement process. The predetermined time period is one hundred milliseconds, which is sufficient to capture and measure motor inrush current of a AC electric motor. Processing for surge current detection and sample accumulation is carried out xe2x80x9cone the flyxe2x80x9d on a sample-by-sample basis, so extra storage and post-acquisition processing are not needed. Once a surge current is detected, a readout of the average or RMS value is available in a fraction of a second.
Other objects, features, and advantages of the present invention will become obvious to those having ordinary skill in the art upon a reading of the following description when taken in conjunction with the accompanying drawings.